Measurement method and system

ABSTRACT

Measurement system for resolving a digital measurement of an unknown analog variable in a noise spectrum in which the spectrum is greater than or less than the least significant weighted value of the digital system. The measurement method of binary approximation is employed until the measuring system indicates once that on a comparison of vote counts the ratio of the unknown analog variable to the binary approximation has a given relationship to a predetermined criteria related to the noise spectrum. Thereafter, an incremental approximation is utilized once the criteria has been initially met to carry out a comparison of the unknown analog variable with an incremented reference and within the predetermined criteria until a voting procedure indicates that the criteria has been successively met for a given number of times. While this voting procedure takes place, the values resulting from the approximations are accumulated for the given number of succesive votes. From this accumulation, the value of the unknown variable is calculated. A look-ahead procedure is also included in the system for accelerating the realization of the initial comparison with the prescribed criteria.

Unite States atent [191 Thompson, deceased et ai.

[4 Dec. 18, 1973 MEASUREMENT METHOD AND SYSTEM international BusinessMachines Corporation, Armonk, NY.

Filed: May 10, 1972 Appl. No.: 252,075

[73] Assignee:

US. CL. 235/L3i, 324/73 AT, 340/347 AD Int. Cl. 03k 13/20 Field ofSearch 235/92 EL, 92 MT, 235/l5l.31; 324/73 R, 73 AT, 73 PC;

[56] References Cited UNITED STATES PATENTS 10/1971 Paine 324/73 AT X7/1972 Prill 340/347 AD [571 Measurement system for resolving a digitalmeasure- Primary Examiner Charles E. Atkinson Assistant Examiner-R.Stephen Dildine, Jr. Attorney-John F. Osterndorf et al.

ABSTRACT ment of an unknown analog variable in a noise spectrum in whichthe spectrum is greater than or less than the least significant weightedvalue of the digital system. The measurement method of binaryapproximation is employed until the measuring system indicates once thaton a comparison of vote counts the ratio of the unknown analog variableto the binary approximation has a given relationship to a predeterminedcriteria related to the noise spectrum. Thereafter, an incrementalapproximation is utilized once the criteria has been initially met tocarry out a comparison of the unknown analog variable with anincremented reference and within the predetermined criteria until avoting procedure indicates that the criteria has been successively metfor a given number of times. While this voting procedure takes place,the values resulting from the approximations are accumulated for thegiven number of succesive votes. From this accumulation, the value ofthe unknown variable is calculated. A

look-ahead procedure is also included in the system for accelerating therealization of the initial comparison with the prescribed criteria.

8 Claims, 4 Drawing Figures DIGITAL Vref. E 3E 21\ I G0 vOrE REFERENCE g29 COUNTER VOLTAGE OAO LOOK AHEAD I VALUE 1 i if A g 1 LOOK AHEAD LOOKAHEAD I m wow COMPARATOR iCOMPARATOR T I l 55 GOINO-GO I MAJORITY e: WELECTRICALLY 24 DECISION I DECISION VARIABLE +4 58 l t DELAY L 52 l l Im...- 1 VOTE I VOTE LENGTH- w COMPLETE COMPARATOR at vows I i t r LENGTHl 55 fgg 22 l NO-GO VOTE TlME DELAY i COUNTER DAC l '-O|O|TAL mt.

SYSTEM STIMULUS m MEASUREMENT LOGIC CONTROL UNIT CHANNEL INTERFACE MUNIT UNDER TEST KEYBOARD FIG. I

:L m I I I I I I I IIIIIIIIJ T I O A M 2 u 2 Y .n m% N w L A N V N T MMH EEL A DDV DC I. RG R R A A A EA A III E I ELI D1 CDH D 6 m ELI. M D m2 mm W W H I H ER E 0T VN Ea? 0W G \M M u W l 2 EL W R OJ E 2 W m mm uwN Dn M H m0 5 m m mm 5 R II R 0M IUNW [LP 00 F N W LC M V0 ZJ VII l I ll l l I I l I I I 1 1 I I I l I I l I ||L 6 i 70 MN l D D 0 0 ZJ EL H IN A H N .I CLT| GN CLE 0 EE .U W HU T M 00 0 AL 0 NB WP NC 0 M M Wm M LPATENTEBOECI 81973 RESET COUNTERS A] TO ZERO LOAD LAV A VLV A2 INTOCOMPARATORS MEASURE saw 2 or 3 TIME GETTref. ,Vref.- FROM EXTERNALSOURCE AND LOAD BOTH DACS I GET MODIFIER FROM EXTERNAL SOURCE AND SAVEENABLE COUNTERS GET Tref. FROM /45 EXTERNAL SOURCE AND LOAD Tref. DAC

SET MODIFIER TO MAXIMUM VALUE I SET Vref. DAC To 47 1/2 MAXVALUE ANDSTART VOTING IS LOOK AHEAD LOST SWITCH SET? YES

IS :15 LOOK 'AHEAD'\\ N0 WON LINE UP? YES SET LOOK AHEAD LOST SWITCH GETGO AND FIG. 3A

,Tb TO A FIG. 3B

NO-GO COUNTS TOB FIG. 5B

PATENTEDUEU sms 3 780,274

' SHEET 3 or 3 SUBTRACT ONE FROM BAD coum SET MODIFIER EQUAL T0 MINIMUMINCREMENT SET ERROR FLAG v RESET AND BAD coum /59 v I sum THEAWSWONOWIJFBITENQACT APPROXIMATION I 61 SUBTRACT ONE LOAD NEW FROM GOODCOUNT vrf. INTO DAC LOAD NEW /62 Tref. INTO DAC CALCULATE 20 BIT ANSWERROUND AND SHIFT RIGHT 4 BITS YES (71- ,SAVE LOW 16 BITS AS FINAL, ANSWERFIG. 3B EXIT MEASUREMENT METHOD AND SYSTEM BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates tomeasurement systems and, more particularly, to a measurement method andapparatus utilizing time or voltage sampling to resolve a digitalmeasurement of an unknown analog variable in the presence of a noisespectrum.

2. Description of the Prior Art The resolution and measurement of highspeed pulse parameters is usually accomplished in an ordinary digitalmeasurement system. A comparator periodically measures two inputs underclock control with one of the inputs variable in accordancewith theresultant output of the comparator. This variable input is typicallyprovided by a digital to analog converter. Such measurement methods inwhich a given initial value is digitally modified by adding to it orsubtracting from it a modifier determined by the sampling are referredto as approximation. When the modifier is divided by two each time it isused, the method is binary approximation.

In the measuring process, a logical decision is made based on thesampling. When the unknown input is greater than the variable input, themodifier is added and when less, it is subtracted. This processcontinues until the modifier has been reduced to a given value, usuallythe least significant bit, enabling the two inputs to approach eachother within the weighted value of the least significant bit of thedigital computer. The value of the modifier at the end of the process isthe ap proximated measurement.

The validity of a measurement made by such a system is governed by thenoise present on the comparator inputs at the instant that a comparisonis made. The effect of the noise is to produce an incorrect algebraicapplication of the modifier and thus cause the value at the'referenceinput to move away from the value at the unknown input rather thantoward it. Since this type of measurement has a statistical probabilityof occurrence and the binary approximation process is terminated bysensing when the modifier has been reduced to a limiting value, theresultant measurement has a statistical precision based on the amount ofnoise relative to the least resolvable digital bit value.

Digital voltmeters are known which operate on these principles and oftenutilize some form of input filtering to reduce the effects of noise.When measurements are required on transient voltages such as the typeoccurring in the testing of high speed digital circuits, filteringcannot be employed.

To improve the measurement precision, the procedure is altered to allowmore than one test sample per modification decision. Thus, for eachvalue of an ap proximation of the reference input, a series of samplesare allowed to occur and the result for each sample is accumulated incounters. One counter is employed where the value of the unknown exceedsthe reference and a second where the value of the reference exceeds theunknown. The number of samples that takes place is usually determined bysensing when either of the counters reaches a predetermined value. Theresult of this procedure is to increase the probability of making thecorrect direction modification as the decision is based on a majority ofvotes rather than a single vote.

SUMMARY OF THE INVENTION The principles of this invention are directedto the further enhancement of the precision of such measurcments bystatistically utilizing the information contained in the values of bothvote counters as well as the majority information. Both time and voltagemeasurements are performed and the result of each sample or vote isaccumulated in go/no-go counters under the control of a logical controlunit responsive to a program sequence contained in a digital computer.Since the effect of noise in a system is to cause a voltage level or apulse transition to vary with time when a measurement is made, theparameter being measured varies over the period of time when it isrequired that it be measured. The principles of the invention assumethat the noise has a normal Gaussian distribution such that the resultof a given number of votes as accumulated in the go/no-go counters is ameasure of the number of occurrences when the instantaneous value of thevariable is above or below the sampling threshold. The ratio of lessertotal votes represents the ratio of a portion of the cumulative noisedistribution to the total distribution.

According to one aspect of the invention, a lookahead procedure ofbinary approximation is first employed accelerating the bringing of thesystem within a first set of system limitations which may be thelimitations of the hardware. By using look-ahead, the speed of responseof the system is enhanced. Once the system is operating within thisfirst set of predetermined limits, measurement is made against a secondset of limits which may be a pre-set criteria related to the noisespectrum. This is also accomplished by binary approximation. After thecriteria is met, the binary approximation is discontinued but anincremental or tracking approximation commences. This approximationcontinues until the pre-set criteria is successively met a predeterminednumber of times. While this predetermined number of votes are measured,the results of the accumulated values of the reference variable arestored. From this cumulative stored value, an indication of the actualvalue of the unknown analog variable is thereafter calculated.

DESCRIPTION OF THE DRAWINGS FIG. l is a block diagram of a configurationof a test system employing the method and apparatus of the invention;

FIG. 2 is a block diagram of a portion of the stimulus measurementsystem of FIG. 1 showing the particular apparatus employed in carryingout the principles of the invention; and

FIGS. 3A and 3B taken together constitute a flow diagram showing theprocess and the control exercised in performing the measurementsaccording to the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thegeneral configuration of a test system includes a process controller 10coupled through a channel interface ill to a test channel includinglogic control unit 12. A stimulus measurement system I3 is coupled tounit 12 and applies the test stimuli to a unit under test 14 and acceptsthe measurement responses from such unit.

Process controller 10, as the primary control of the test system, maytake the form of an IBM 1802 process controller which is coupled toinput/out devices such as a card reader 15, keyboard 16, printer 17 andstorage files 18 of the magnetic tape or disc type. Process controllermay be connected by a communications link with an off-line dataprocessing system such as one of the models of the IBM System 360.Communication from process controller 10 to the test channel takes placethrough interface 11 which may be an IBM l826 channel interface. Theinput/output devices are standard for such a test system and they act toprocess the input/out data while the test channel performs the tests.

Logic control unit 12 is a microprogrammable logic and control unit thatis test dependent. It may take the form of the central processing unitof an IBM System 360 Mod 30. Dependent on the microprogramming suppliedto unit 12, a particular type of testing is performed. Stimuli aregenerated in system 13 which are supplied to unit under test 14. Theconnections are made to the input/output connectors of the unit whichmay be a semiconductor integrated circuit. Connection from system 13 tothe unit under test is ordinarily achieved through a handler which movesthe unit to be tested into a test station for application of the stimuliand measurement of the responses and thereafter accomplishes a sortingoperation dependent on the results of the test.

One such measurement response connection is then effected with anapproximated reference value.

Comparator is coupled to a reference voltage digital to analog converter(DAC) 21 and a time delay digital to analog converter (DAC) 22 throughan electrical variable delay 23 which is varied according to a clocksignal at 24. Delay 23 accepts the output from time delay DAC 22 andcontrols the timing when comparator 20 makes a measurement after theoccurrence of clock pulse 24. The reference values are supplied to thedigital to analog converters 21, 22 from logic control unit 12 throughlines 25, 26.

The comparator provides outputs to go and no-go vote counters 27, 28which in turn provide a go count at 29 and a no-go count at 30 to thelogic control unit. These counts are also provided to look-aheadcomparator 31, votelength comparator 32 and majority decision circuits33. A predetermined look-ahead value (LAV) is loaded into comparator 31from logic control unit 12 through line 34 and a predeterminedvotelength value (VLV) is loaded into votelength comparator 32 throughline 35. The measurement circuitry provides three separate outputindications: a look-ahead won indication is provided at line 36 fromcomparator 31; a vote complete indication on line 37 from comparator 32;and a go/no-go decision on line 38 from circuit 33.

In the performance of the test method, the first step is to reset thelook ahead lost switch and the counters 27, 28 to zero (Block 41). Thenthe look-ahead value and votelength value are loaded into comparators31, 32 (Block 42). Next, a determination is made as to whether themeasurement is to be in the time or voltage mode (Block 43). If the modeis time, then the time and voltage input reference values are loadedinto reference voltage digital to analog converter 21 through line 25and time delay digital to analog converter 22 through line 26 (Block44). If the mode is voltage, then the reference value for time only isobtained and loaded into time delay digital to analog converter 22through line 26 (Block 45).

It is to be understood that all of these external measurement criteria,or prescribed criteria, that is votelength value, look-ahead value, timeand voltage reference values are provided to the measuring apparatusfrom logic control unit and in turn from process controller 10. For aparticular part number that is the unit under test, these values may bestored in files 18 ir keyed into the process control system throughkeyboard 16. Once the external measurement criteria are provided tologic control unit 12 and then to stimulus measurement system 13, theprocess controller 10 may be disconnected from the test channel untilthe completion of the test operations. The process controller operatesunder the control of a pre-loaded program or it may be responsive tooperator action.

If it is assumed that the measurement mode is a voltage mode and thatthe time reference has been loaded into time digital to analog converter22 (Block 45), the voltage is measured on the unknown input waveform Einat the time specified by the time reference value. Logic control unit 12calculates the modifier for the voltage reference from the previouslysupplied external measurement criteria and places this value intodigital to analog converter 21. This is the step shown in Block 46. Thefirst approximation in such a measurement mode is one-half of thevoltage measurement full scale range (Block 47).

The system is now prepared to begin voting (Block 48) and counters 27,28 are enabled and permitted to count comparisons provided fromcomparator 20 to counters 27 28. When the comparison determines that Einis greater than the approximated value, a count is recorded in Go VoteCounter 27. Similarly, when Ein is less than the approximated value, acount is recorded in No Go Counter 28. When the counts are such that thevalues loaded into either the look-ahead or votelength comparators 31,32 is satisfied as indicated on lines 36, 37 voting is terminated. Ifneither of these values is satisfied, the voting continues (Block 49).When completed, an interrogation is made (Block 50) as to whether thelook-ahead lost switch has been set. If it is assumed that look-aheadcomparator 31 is satisfied first it indicates that one of the counters27, 28 has counted to a predetermined value which is normally much lessthan its full scale count while the other counter still has a zerocount. The predetermined value is the look-ahead value provided on line34 to lookahead comparator 31. The effect of this measurement is toindicate that the reference values provided to comparator 20 are stillsignificantly far away from the input voltage value Ein provided on line19, such that the measurement is not being made within the noise region.By the indication provided on line 36, the logic control unit isinformed that a measurement has been completed.

If the look-ahead lost switch set interrogation (Block 50) indicatesthat the switch has been set, the logic control unit then requests thego and no-go counts provided from counters 27, 28 on lines 29 and 30(Block 53). Interrogation is made as to whether either count is zero(Block 54). If the look-ahead lost switch was not set (Block 50),determination is made if line 36 is up (Block 51). If not, thelook-ahead lost switch is set (Block 52) and then the go and no-gocounts are provided to logic control unit 12 (Block 53). If theindication from the interrogation in Block Sll (look-ahead won) ispositive, logic control unit 112 continues the binary approximationprocedure. The modifier is halved and a determination made as to whetherit is to be added or subtracted. This determination is made based on thelevel on Go/ No-Go decision line 38.

If the go and no-go counts have been obtained, determination is made asto whether either count is equal to zero (Block 54). If neither countequals zero, a check is made against the prescribed criteria (Block 55).If not met, the binary approximation continues. When the prescribedcriteria has been initially met, the mode of measurement is changed.However, if either count is equal to zero again indicating that thelook-ahead interrogation is positive, the binary approximation continuesas provided in Block 56. A count of one is subtracted from the bad countand if the resultant is equal to zero (Block 57) an error flag isset andexit is made from the test procedure (Block 58). On the other hand, ifthe bad count does not equal zero, a new modifier which is one-half ofthe previous approximation is added to or subtracted from theapproximation (Block 59). The determination of whether the change is tobe additive or subtractive is made in majority decision circuit 33 whichcompares the counts in go and no-go counters 27, 28. When the count incounter 27 exceeds that in counter 25;, the change is additive.Correspondingly, when the count in counter 28 exceeds the count incounter 27, the change is subtractive.

If it is assumed that the decision is to subtract the next modification,digital to analog converter 21 is loaded through line 25 with the newmodifier and another comparison is made in comparator 20. The countersare reset and the voting procedure commences as in Block 43. These stepsof the process require an interrogation for a time measurement (Block60) and the loading of the appropriate time or voltage reference intoits corresponding digital to analog converter (Blocks 6E, 62). Thisapproximation technique continues. Each successive time that a vote iscarried out, the previous modification value is halved until thelook-ahead determination is no longer positive and a comparison is madewith the pre-set criteria as provided in Block 55 after the votelengthvalue provided to votelength comparator 32 on line 35 is matched by thecounts in counters 27, 28.

Logic control unit 112 is notified by voting complete line 37 to checkthe values in go counter 27 and no-go counter 28 and to compare them todetermine if the ratio is greater than or equal to the pre-specifiedcriteria (Block 55). Assuming that the criteria is not satisfied, theprocess continues by moving to Block 56. The binary approximationprocedure continues halving the modifier each time. At each evaluationof the counters when vote length wins as indicated on line 37 andlookahead does not as indicated on line 36, the two counts are comparedto see if they match the criteria.

As soon as counters 27, 28 meet the criteria, it means that thereference value inputted into the comparator is within the noise band ofthe input voltage Ein to be measured. The procedure follows the path toBlock 63 in FIG. 3B. A modifier is set which is equal to the minimumresolution of the system and the procedure changes from a binaryapproximation method to an incremental or tracking approximation method.The bad count is reset (Block 64). For each succeeding measurement whichmeets the criteria (Block 55) the digital voltage value fed to referencevoltage digital to analog converter H is also added in an accumulator inlogic control unit 12 (Block 65) until a prespecified number ofsuccessful measurements or votes have been accumulated.

After each successful measurement is completed, the good count isdecremented by one (Block 66) and a determination is made as to whetherthe good count equals zero (Block 67). If the good count does not equalzero, the measuring process continues using the incremental or trackingapproximation method by first interrogating as to whether themeasurement is for time or voltage (Block 66). Assuming that the goodcount equals zero, the value of Ein is determined as the answer to beobtained from the procedure. To accomplish this, the value stored in theaccumulator is divided by the total number of measurements that weremade.

As shown in FIG. 38 by way of illustration, a 20 bit answer, 16 bitmeasurements can be calculated (Block 68), followed by a determination(Block 69) as to the degree of precision required from the procedure. Ifhigh precision is not required, the 20 bit answer is rounded and shiftedright four bits (Block 70). If precision is required, the lowest order16 bits are saved as the final answer (Block 71). In both instances,exit is made from the system through channel interface 11, processcontroller 10, to be provided at an output device such as the printer 17(Block 72).

If any measurement, during the series of measurements after the criteriahas once been met, does not match the criteria, then the voltagereference digital value is incremented by the least significant bit ofthe system. The process continues according to incremental approximationmethod until a predetermined number of good counts is obtained.

The same sequence of events takes place if the measurement modedetermined in Block 43 is to measure time. However, in this type ofmeasurement, the reference values are loaded into the voltage referenceand time delay digital to analog converters 21, 22 (Block 44). For atime measurement, the first approximation is specified by input data.Thus, as in Block 47A, the modifier is obtained from the external sourceand provided through process controller 10 and channel interface 111 tologic control unit 12.

As there is always the possibility that the input value is not withinthe range of the measurement system, a counter is provided referred toas a bad count counter to evaluate the number of times it has tried tomake a measurement and has not achieved a successful count. The systemceases to count and sends out a signal indicating an error and also anindication that it cannot determine the value it is seeking (Block 58).

A method and apparatus have been described for accomplishingmeasurements on the value of a voltage or the time of transition. Toperform these measurements, the first approximation, first modificationand the reference levels are constants derived from the analysis ofinput data and contained in a storage system such as the storage file18. The first approximation and reference level are transmitted to themeasuring apparatus with a start vote command. A look-ahead value andvote length value have already been loaded into the measuring apparatus.Look-ahead is a voting method defined by having accumulated theprescribed value in either counter and none in the other counter. Whenthe voting is completed, the results of it are transmitted to the logiccontrol unit 12. lf look-ahead won is indicated on line 36, a newapproximation is computed, based on a binary approximation method. Thevoting process is repeated until eventually votes are accumulated in thego/no-go counters 27, 28. When both counters contain a value other thanzero, voting is terminated when either counter reaches the prescribedvote length as indicated by the value provided on line 35 and theindication on line 37. Analysis is performed on the values contained inthe counters to determine if their difference is less than thepredetermined criteria which was loaded into the system as a systemconstant. When the vote difference meets the criteria, the presentapproximation is within a given range of the mean of the noisedistribution and thus within a given range of the value of the parameterbeing measured. The operation is changed from the binary approximationmethod to an incremental or tracking approximation method which enhancesthe accuracy of the measuring process.

Three significant things occur. The modifier is changed to a constantvalue rather than halving the previous value; the then present value ofthe then approximation is added to an accumulated value and a registerpreviously loaded with a good count is decremented by one. The newapproximation value, which is adjusted by a constant, is provided to themeasuring system and the voting commences again. Analysis of the valuesin the counters is performed each time that voting is stopped becauselook-ahead won or because a maximum vote length was achieved.Determination is made if the prescribed criteria has been met. When itdoes, the then present value of the approximation is again added to theaccumulator and the good counter is decremented. If the values in thecounters do not meet the criteria, there is no addition of thatapproximation to the accumulator and a new approximation is computed tocontinue the voting. The process is continued until the good counter isdecremented to zero terminating the measuring process.

When the measuring process terminates, the accumulator contains the sumof all values of the measurements meeting the criteria. This sum isdivided by the number of good entries to average the approximationsmeeting the criteria. The criteria against which the measurements takeplace is related to the system noise. By proper selection of the systemconstants, a final measurement is arrived at, in the presence ofsignificant amounts of noise, which provides an accurate indication ofthe parameter being measured.

In performing the measurements, conditions may occur which preventnormal termination of the method. For example, if during the binaryapproximation, there is a failure to meet the criteria because theparameter is not present or is not within a reasonable field ofmeasurement, a bad counter is decremented. This counter is decrementedeach time a vote did not meet the criteria. The occurrence of a goodcount resets this counter with a new value. When the bad" counterdecrements to zero, a signal is provided to the process controller toprovide an output indication.

Another condition which may occur takes place when the noise band isless than that which could be resolved by the least value ofmodification achievable by the system. The value of the measurement isbracketed but the available approximation of values cannot be resolved.To avoid this condition, the successive sign reversals of the new valueof the modifier are counted. After a predetermined number of thesereversals have occurred, one of the values about which the approximationoscillates is accepted as a good value and the voting proceeds in anormal manner.

The method of measurement that has been described first provides forsearching to find an unknown signal using a binary approximation. Thenas thesignal noise region is entered, an incremental approach is usedwith a preset criteria to remain centered within the noise region.During this latter portion of the process, accumu lation is made of anumber of measurement results and digital averaging takes place toimprove the resolution. The effect of the digital averaging is to permitsmoothing of time measurements.

Such a test system method may be modeled in software such as Fortran andrun on a typical general purpose digital computer. It may also beperformed in discrete hardware such as that found in an IBM System 360.

The results of one such modeling program are set forth in Table l forvarious values of vote length and DEL (the difference between the votelength and the number of losses which determines the vote ratiocriteria).

TABLE 1 Measurement Resolution with 50 Picosecond Noise Average StandardVote Ratio Sample Error Length DEL Criteria Count Deviation picosecond l30 37.2 8 92 12.8 8 0 0 263 5.7 8 7 01 ll 240 15.9 l6 l4 0.lll 463 11.832 28 0.1 ll 920 9.4 64 56 0.l ll 1858 8.6 8 6 0.200 25l 8.4 l6 12 0.200472 6.5 32 24 0.200 943 5.9 64 48 0.200 I904 5.0 8 4 0.333 3l6 50 lo 80.333 540 3.7 32 16 0.333 1004 3.4 64 32 0.333 l986 2.4 8 3 0.384 3763.7 l6 6 0.384 677 2.8 32 12 0.334 H2) 2.2 64 24 0.384 2039 L8 8 2 0.428558 3.0 16 4 0.428 2.4 32 8 0.428 1459 [.3 64 lo 0.428 2390 L2 8 I 0.4661269 2.0 l6 2 0.466 I817 I5 32 4 0.466 2788 L0 64 8 0.466 4372 0.6

In all cases, the look-ahead value was four and the number of goodvalues to be averaged was 16 (except the first two entries). Thestatistical sample size is 66 and the assumed noise is lF 50 ps. Thefirst two entries are used for reference and are obtained by making thegood counter equal to one thus representing results which would beobtained if the algorithm were not known analog quantity with respect toknown quantity in the presence of an unwanted parameter by measurementapparatus coupled to a computing system having said known quantity, ameasuring reference and a prescribed criteria related to said parameterall stored therein, comprising the steps of used. The results for a votelength 1 represents the most elementary binary search method withoutmajority voting; the results for a vote length 8 represents the binarysearch with majority voting. The average sample count is the statisticalaverage of the number of samples required to approximate the measurementand, for a given system, is a relative measure of the time required toproduce a measurement. The standard error deviation is the statisticaldeviation (Id) of the error computed by the modeling program where theerror is defined as the difference between a value representing themodeled transition and the modeled measurement. The smaller values ofthe deviation indicate a higher, and therefore, more desirable precisionof measurement. Measurements with higher precisions require largersamples.

While this invention has been particularly described What is claimed is:l. A method for determining the value of an un loading said knownquantity, measuring reference and prescribed criteria and a firstapproximation of said unknown quantity into said measurement apparatus,

performing a first set of successive comparisons between saidapproximation and unknown quantity with respect to said known quantityfor said measuring reference until said criteria is satisfied, each suchsuccessive comparison after the first being made with a digitallymodified approximation of the preceding approximation,

performing a second set of successive comparisons between said lastdigitally modified approximation and unknown quantity with respect tosaid known quantity for a predetermined number of said measuringreference, each such successive comparison being made with a trackingmodified approximation of the preceding approximation, and

calculating the value of said unknown quantity from the results of saidsecond set of comparisons.

2. In the method of claim I, wherein the unknown analog quantity is avoltage which is measured at a fixed cyclically recurring time loadedinto said apparatus, said measurement being made in the presence ofnoise and in which in said first set of comparisons a digitally modifiedapproximation of said voltage is successively compared with said voltageuntil said criteria is satisfied and in which said second set ofcomparisons is performed by track approximating the value of saidvoltage.

3. In the method ofclaim 1, wherein the unknown analog quantity is thetime at which a fixed value of voltage occurs in the presence of noise,said fixed value of voltage being loaded into said apparatus and inwhich said first set of comparisons are performed with a digitallymodified approximation of said time by successive comparisons with saidtime until said criteria is satisfied and in which said second set ofcomparisons is performed by track approximating said time.

4. In the method of claim 1, wherein the first set of successivecomparisons are made by binary digital approximations of said unknownquantity.

5. in the method of claim 1, wherein the second set of successivecomparisons are made by incremental approximations of said unknownquantity.

6. In the method of claim 5 wherein said measuring reference is a votelength with each vote being the equivalent of a comparison made betweenthe unknown analog quantity and an incremented approximation of saidquantity, said vote length being achieved for a predetermined number oftimes and wherein said method further comprises the step of accumulatingthe values of all of said incremented approximations from which thevalue of said quantity is calculated.

7. In the method of claim 1 and further comprising the steps of loadinga second measuring reference into said apparatus, said second referencebeing related to the measuring limitations of said apparatus, andperforming a third set of comparisons between said first approximationand unknown quantity prior to said first set of comparisons as long assaid second reference is satisfied.

8. In the method of claim 7, wherein said second reference is a lookahead value provided by said computing system for reducing the number ofcomparisons to be performed by shortening the time to bring saidapproximation within the measuring limitations of said apparatus, saidlook ahead value being satisfied when all of the comparisons of saidthird set are of one type.

1. A method for determining the value of an unknown analog quantity withrespect to known quantity in the presence of an unwanted parameter bymeasurement apparatus coupled to a computing system having said knownquantity, a measuring reference and a prescribed criteria related tosaid parameter all stored therein, comprising the steps of loading saidknown quantity, measuring reference and prescribed criteria and a firstapproximation of said unknown quantity into said measurement apparatus,performing a first set of successive comparisons between saidapproximation and unknown quantity with respect to said known quantityfor said measuring reference until said criteria is satisfied, each suchsuccessive comparison after the first being made with a digitallymodified approximation of the preceding approximation, performing asecond set of successive comparisons between saiD last digitallymodified approximation and unknown quantity with respect to said knownquantity for a predetermined number of said measuring reference, eachsuch successive comparison being made with a tracking modifiedapproximation of the preceding approximation, and calculating the valueof said unknown quantity from the results of said second set ofcomparisons.
 2. In the method of claim 1, wherein the unknown analogquantity is a voltage which is measured at a fixed cyclically recurringtime loaded into said apparatus, said measurement being made in thepresence of noise and in which in said first set of comparisons adigitally modified approximation of said voltage is successivelycompared with said voltage until said criteria is satisfied and in whichsaid second set of comparisons is performed by track approximating thevalue of said voltage.
 3. In the method of claim 1, wherein the unknownanalog quantity is the time at which a fixed value of voltage occurs inthe presence of noise, said fixed value of voltage being loaded intosaid apparatus and in which said first set of comparisons are performedwith a digitally modified approximation of said time by successivecomparisons with said time until said criteria is satisfied and in whichsaid second set of comparisons is performed by track approximating saidtime.
 4. In the method of claim 1, wherein the first set of successivecomparisons are made by binary digital approximations of said unknownquantity.
 5. In the method of claim 1, wherein the second set ofsuccessive comparisons are made by incremental approximations of saidunknown quantity.
 6. In the method of claim 5 wherein said measuringreference is a vote length with each vote being the equivalent of acomparison made between the unknown analog quantity and an incrementedapproximation of said quantity, said vote length being achieved for apredetermined number of times and wherein said method further comprisesthe step of accumulating the values of all of said incrementedapproximations from which the value of said quantity is calculated. 7.In the method of claim 1 and further comprising the steps of loading asecond measuring reference into said apparatus, said second referencebeing related to the measuring limitations of said apparatus, andperforming a third set of comparisons between said first approximationand unknown quantity prior to said first set of comparisons as long assaid second reference is satisfied.
 8. In the method of claim 7, whereinsaid second reference is a look ahead value provided by said computingsystem for reducing the number of comparisons to be performed byshortening the time to bring said approximation within the measuringlimitations of said apparatus, said look ahead value being satisfiedwhen all of the comparisons of said third set are of one type.